Why is part of a player's ability bars grayed?

The importance of each ability depends on your team's strategy and the player's position on the field. You can see each position's "profile" on the strategy page.

If a player does not have the right profile for his current position, certain of his abilities might be higher than the required values. In this case, the exceeding part is grayed to show that it does not improve his performance in the current position.

How does the defense and attack cursor influence strategy?

In neutral position, your team will play normally. In attack position, your team's attack will be favored and your defense slightly diminished . In defense position, your team's attack will be slightly weaker but your defense will be better.